Measuring the rate of extrusion of a molten resin though a capillary of known size under a known applied pressure is a widely accepted method of determining rheological properties of thermoplastic polymers. These test devices are known in the art as capillary rheometers. They comprise a sample cylinder that is generally equipped with a die having a capillary bore drilled in it. There are two common types of capillary rheometers which are distinguished by the means used to force the molten polymer contained in the sample cylinder through the capillary. In one design, the cylinder and die are immersed in a heated oil tank. A stirrer is provided to circulate the oil. The die is fastened to a hole in the bottom of the tank to prevent oil leakage but allow extrusion of a sample through the capillary. Heating the oil in the tank heats the cylinder which heats a polymeric sample retained in it. This type of capillary rheometer uses a pressurized gas to eject the molten sample through the capillary and is much preferred for testing viscous filled resins.
The second type employs a metal piston which fits tightly in the sample cylinder bore and pushes the polymer through the capillary. The whole unit is enclosed in a muffle tube furnace for heating. Such rheometers work well at relatively low shear rates and temperatures for pure, stable polymer resins. However, they do not work well for highly filled polymers tested at relatively high extrusion pressures and temperatures. Measurements may be inaccurate because of friction between the piston and the cylinder wall which is aggravated by the presence of fillers such as fiberglass.
Both types of conventional rheometers have characteristics that make them difficult to use. For example, it is difficult to control oil temperature at test temperatures above about 200.degree. C. It is also extremely difficult to clean the sample cylinders and capillary dies without completely disassembling the apparatus. This leads to long cycle times between measurements. The apparatus must also be disassembled to change the capillary die as its size changes due to abrasion by fillers. The preferred way of cleaning sample cylinders and dies is heating them in a furnace or immersing them in a molten salt bath to burn away the polymer. This also requires complete disassembly of conventional rheometers.
Accordingly, it is an object of this invention to provide an improved capillary rheometer that is more accurate and easily serviceable.